The present invention relates to an optically anisotropic sheet comprising aligned discotic liquid crystal molecules, The invention also relates to a polarizing plate comprising an optically anisotropic layer, an orientation layer, a transparent support, a polarizing film and a transparent protective film in this order. The invention further relates to a process for orienting discotic liquid crystal molecules with an orientation layer.
A liquid crystal display comprises a liquid crystal cell, a polarizing element and an optical compensatory sheet (phase retarder). In a liquid crystal display of transmission type, two polarizing elements are arranged on both sides of the liquid crystal cell and one or two optical compensatory sheets (phase retarders) are arranged between the liquid crystal cell and the polarizing elements. On the other hand, a liquid crystal display of reflection type comprises a reflection plate, a liquid crystal cell, an optical compensatory sheet and a polarizing element in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecules. The alignment of the rod-like liquid crystal molecules is determined according to a display mode of the liquid crystal cell. Various display modes of the liquid crystal cell have been proposed. Examples of the mode for transmission type include TN (Twisted Nematic) mode, IPS (In-Plane Switching) mode. FLC (Ferroelectric Liquid Crystal) mode, OCB (Optically Compensatory Bend) mode, STN (Super Twisted Nematic) mode, VA (vertically Aligned) mode and ECB (Electrically Controlled Birefringence) mode. Examples of the mode for reflection type include TN mode, HAN (Hybrid Aligned Nematic) mode and GH (Guest-Host) mode.
The optical compensatory sheet has functions of removing undesired color from a displayed image and enlarging a viewing angle. As the optical compensatory sheet, a stretched birefringent film has been conventionally used.
Recently, an optical compensatory sheet comprising an optically anisotropic layer on a transparent support has been proposed in place of the stretched birefringent film. The optically anisotropic layer is formed from liquid crystal molecules. Since the liquid crystal molecules have various alignment forms, an optical compensatory sheet obtained by using the liquid crystal molecules has specific optical characteristics that cannot be obtained by the conventional stretched birefringent film.
The optical characteristics of the optical compensatory sheet are determined according to optical characteristics (i.e., display mode) of the liquid crystal cell. Various optical compensatory sheets properly applied for various display modes can be produced by using liquid crystal molecules. As the liquid crystal molecules for optical compensatory sheet, rod-like or discotic liquid crystal molecules are generally used.
Various optical compensatory sheets using discotic liquid crystal molecules according to various display modes have been disclosed. For example, an optical compensatory sheet for TN mode is disclosed in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. An optical compensatory sheet for IPS or FLC mode is disclosed in Japanese Patent Provisional Publication No. 10(1998)-54982, and a sheet for OCB or HAN mode is disclosed in U.S. Pat. No. 5,805,253 and International Patent No. W096/37804. Further, a compensatory sheet for STN mode is disclosed in Japanese Patent Provisional Publication No. 9(1997)-26572, and one for VA mode is disclosed in Japanese Patent No. 2,866,372.
In the optical compensatory sheet for STN mode disclosed in Japanese Patent Provisional Publication No. 9(1997)-26572, the discotic liquid crystal molecules are aligned at an average inclined angle of 50xc2x0 to 90xc2x0 (namely, the liquid crystal molecules are vertically aligned).
An optically anisotropic sheet comprises discotic liquid crystal molecules aligned at an average inclined angle of 50xc2x0 to 90xc2x0. In the optically anisotropic sheet, an average direction of optical axes (normal lines to the discotic planes) of the molecules is generally perpendicular to a rubbing direction of the orientation layer. The optically anisotropic sheet is practically produced in the form of a roll. It is the easiest way to conduct a rubbing treatment along a longitudinal direction of the rolled sheet. Accordingly, the most easily produced optical compensatory sheet having an optically anisotropic layer in which the discotic liquid crystal molecules are aligned at an average inclined angle of 50xc2x0 to 90xc2x0 has an average direction of optical axes perpendicular to the longitudinal direction (i.e., the average direction of optical axes is parallel to the lateral direction).
In a polarizing element having stretched polymer film, the transparent axis is perpendicular to the stretching direction. The polarizing element is also practically produced in the form of a roll. It is the easiest way to stretch the film along the longitudinal direction of the roll. Accordingly, the most easily produced polarizing element has a transparent axis perpendicular to the longitudinal direction (i.e., the transparent axis of that polarizing element is parallel to the lateral direction).
Where the rolled optically anisotropic sheet is laminated with the rolled polarizing element, the optical axes of discotic liquid crystal molecules and the transparent axis of the element are easily so arranged that the optical axes are essentially parallel to the transparent axis.
However, the optical axes and the transparent axis of the element are preferably so arranged in some display modes that the optical axes are essentially perpendicular to the transparent axis.
The discotic liquid crystal molecules must be so aligned that the optical axes of liquid crystal molecules are parallel to the rubbing direction of the orientation layer to arrange the optical axes along the longitudinal direction of the rolled sheet. In the present specification, the words xe2x80x9cthe optical axes of discotic liquid crystal molecules are parallel to the rubbing direction of the orientation layerxe2x80x9d mean that an average direction of lines obtained by projecting the normal lines to the discotic planes of the molecules on the support is parallel to the rubbing direction. A new orientation layer having such function is required to orient the optical axes of the discotic liquid crystal molecules parallel to the rubbing direction. A conventional orientation layer orients the optical axes of the discotic liquid crystal molecules perpendicularly to the rubbing direction.
An object of the present invention is to provide an optically anisotropic sheet in which discotic liquid crystal molecules are so aligned that the optical axes are parallel to the rubbing direction.
Another object of the invention is to provide a polarizing plate in which the optical axes of discotic liquid crystal molecules are easily arranged essentially perpendicularly to the transparent axis of polarizing film.
A further object of the present invention is to align discotic liquid crystal molecules so that the optical axes are parallel to the rubbing direction.
The present invention provides an optically anisotropic sheet comprising an optically anisotropic layer formed from discotic liquid crystal molecules, an orientation layer subjected to rubbing treatment and a transparent support in this order, wherein the discotic liquid crystal molecules are so aligned that an average inclined angle of discotic planes of the discotic liquid crystal molecules is in the range of 50xc2x0 to 90xc2x0 and that an average direction of optical axes of the discotic liquid crystal molecules is essentially parallel to a rubbing direction of the orientation layer.
The invention also provides a rolled polarizing plate comprising an optically anisotropic layer formed from discotic liquid crystal molecules, an orientation layer subjected to rubbing treatment, a transparent support, a polarizing film and a transparent protective film in this order, wherein the discotic liquid crystal molecules are so aligned that an average inclined angle of discotic planes of the discotic liquid crystal molecules is in the range of 50xc2x0 to 90xc2x0 and that an average direction of optical axes of the discotic liquid crystal molecules is essentially parallel to a rubbing direction of the orientation layer.
The invention further provides a process for orienting discotic liquid crystal molecules, comprising the steps of: coating a solution of a copolymer comprising repeating units represented by the formula (I) and repeating units represented by the formula (II) on a support to form a coated layer; rubbing a surface of the coated layer to form an orientation layer; coating a solution containing discotic liquid crystal molecules on the orientation layer to orient the discotic liquid crystal molecules so that an average inclined angle of discotic planes of the discotic liquid crystal molecules is in the range of 50xc2x0 to 90xc2x0 and that an average direction of optical axes of the discotic liquid crystal molecules is essentially parallel to a rubbing direction of the orientation layer: 
in which each of R1 and R2 independently is hydrogen, a halogen atom or an alkyl group having 1 to 6 carbon atoms; M is an alkali metal ion; L1 is a divalent linking group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NHxe2x80x94, an alkylene group and a combination thereof; each of L2, L3 and L4 independently is a single bond or a divalent linking group selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NH, xe2x80x94SO2xe2x80x94, an alkylene group, an alkenylene group, an alkynylene group and a combination thereof; each of Ar1, Ar2, Ar3 and Ar4 independently is an aromatic ring, which can have a substituent group; each of m and n independently is 0 or 1; x is 10 to 95 mole %; and y is 5 to 90 mole %.
In the present specification, the term xe2x80x9cessentially perpendicularxe2x80x9d or xe2x80x9cessentially parallelxe2x80x9d means that the angle between the noticed directions is within the range of 90xc2x0xc2x15xc2x0 or 0xc2x0xc2x15xc2x0, respectively. The allowance of the angle is preferably less than xc2x14xc2x0, more preferably less than xc2x13xc2x0, further preferably less than xc2x12xc2x0, and most preferably less than xc2x11xc2x0.
The present inventors have succeeded in aligning discotic liquid crystal molecules, whose optical axes are parallel to the rubbing direction. In the prepared optically anisotropic sheet, discotic liquid crystal molecules are so aligned that the optical axes are essentially parallel to the rubbing direction. A rolled optically anisotropic sheet comprising discotic liquid crystal molecules whose optical axes are parallel to the longitudinal direction can be easily produced.
Further, a polarizing element having a transparent axis perpendicular to the longitudinal direction (i.e., along the lateral direction) is most easily produced. Therefore, a polarizing plate in which the optical axes of discotic liquid crystal molecules are essentially perpendicular to the transparent axis of the polarizing film can be easily produced by simply laminating the rolled optically anisotropic sheet and the rolled polarizing element of the invention with their rolled forms maintained.
According to the invention, it is easy to arrange the optical axes of discotic liquid crystal molecules essentially perpendicularly to the transparent axis of the polarizing film.